1. Field of the Invention
This invention relates a matrix substrate and a liquid crystal display apparatus and, more particularly, it relates to a matrix substrate and a liquid crystal display apparatus featured by horizontal and vertical shift registers to be used for scanning liquid crystal devices for display operations as well as to a projector using the same.
2. Related Background Art
In recent years, display apparatus have been playing ever-increasing important roles as means of multi-media communication using images, sounds and written texts. Of them, liquid crystal display apparatus have the advantage of being very thin and consuming little power and the industry of manufacturing liquid crystal display apparatus has grown to a major industry that is comparable with the semiconductor manufacturing industry. It is expected that liquid crystal display apparatus are used in the future not only for personal computers but also for work stations and home television sets having a large display screen. However, a large liquid crystal display apparatus having a large screen is accompanied by high manufacturing cost and electric requirements to be met to drive its large screen. Normally, the manufacturing cost of a liquid crystal display apparatus increases as a function of the square to the cube of the size of the display screen.
In an attempt to bypass this problem, projection systems adapted to optically enlarge the image formed on a relatively small liquid crystal display screen for viewing have been attracting attention. Such a system has become feasible due to the recent technological development that has made it possible to manufacture micro-semiconductor devices on a mass production basis to exploit the scale merit. Then, in the liquid crystal display panel is of the TFT type, TFTs that are small and have a sufficient drive effect have to be used. Additionally, for technological reasons, TFTs using amorphous Si are being replaced by those using polycrystalline Si. On the other hand, video signals for the level of resolution required to meet the NTSC standards or other ordinary television standards do not necessarily have to be processed at high speed.
Thus, it is now possible to produce a liquid crystal display apparatus, wherein not only the TFTs but also the peripheral drive circuits such as shift registers and decoders are made of polycrystalline Si so that the display region and the peripheral drive circuits may be formed integrally. However, polycrystalline Si is not as good as single crystalline Si and, for producing a liquid crystal display apparatus of the XGA (extended Graphic Array) class or the SXGA (Super extended Graphics Array) class, as expressed in terms of the standards for the resolution of computer-generated graphic images, shift registers and other devices may have to be divided and arranged at a plurality of locations. Then, the junctions of adjacent devices can generate noises referred to as ghosts, which provide a problem to be dissolved in this field of technology.
On the other hand, display apparatus comprising a single crystalline Si substrate that shows a much higher drive force that display apparatus having an integral structure of polycrystalline Si has been attracting attention. Since the transistors of the peripheral drive circuits of such display apparatus show a satisfactory drive force and hence do not have to be divided, they are free from the problem of noises.
Regardless of polycrystalline Si or single crystalline Si, it is possible to provide a reflection type liquid crystal display apparatus comprising liquid crystal devices and realized by connecting the drains of TFTs to respective reflection electrodes and arranging pieces of liquid crystal respectively pinched by the reflection electrodes and a corresponding transparent common electrode to form reflection type liquid crystal devices, which liquid crystal devices are then scanned by means of horizontal and vertical shift registers arranged on a same semiconductor substrate.
The applicant of the present patent application has disclosed in Japanese Patent Application Laid-Open No. 9-73103 a reflection type liquid crystal display apparatus realized by using polycrystalline Si and single crystalline Si. A liquid crystal display apparatus as disclosed in the above patent document is proposed to solve some of the problems of known liquid crystal display apparatus of the type under consideration including that light entering the pixel electrodes are scattered in various directions by the undulations on the surface thereof to remarkably reduce the reflection efficiency of light and that such undulations on the surface of the pixel electrodes can give rise to a defective orientation in the process of rubbing the oriented film conducted in the course of mounting the liquid crystal to consequently produce a defective orientation in the liquid crystal that can degrade the quality of the displayed image due to a poor contrast.
According to the above cited Japanese Patent Application Laid-Open No. 9-73103, the surface of the pixel electrodes is polished by means of a technique of chemical mechanical polishing (referred to as CMP hereinafter). Then, all the surfaces of the pixel electrodes are made mirror plane and flush with each other.
Now, an active matrix substrate and a method of manufacturing the same will be summarily described by referring to FIGS. 23A to 23E and 24F to 24H of the accompanying drawings. Note that, while FIGS. 23A to 23E and 24F to 24H show only part of the pixel section of an active matrix substrate, peripheral drive circuits including shift registers for driving the switching transistors of the pixel section may also be formed on the same substrate.
Firstly, an n-type silicon semiconductor substrate 201 showing an impurity concentration level of not greater than 1015 cmxe2x88x923 is partly and thermally oxidized to produce a LOCOS 202 for each pixel and then boron ions are implanted to a dosage level of 1012 cmxe2x88x922, using the LOCOS 202 as mask, to produce a PWL 203 which is a p-type impurity region showing an impurity concentration level of about 1016 cmxe2x88x923 The substrate 201 is then thermally oxidized once again to produce a gate oxide film 204 having a film thickness of not greater than 1,000 angstroms (FIG. 23A).
After forming a gate electrode 205 of n-type polysilicon doped with phosphor to a concentration level of about 1020 cmxe2x88x923, phosphor ions are implanted into the entire surface of the substrate 201 to a dosage level of about 1012 cmxe2x88x922 to produce an NLD 206 which is an n-type impurity region showing an impurity concentration level of about 1016 cmxe2x88x923 and subsequently phosphor ions are implanted to a dosage level of about 1015 cmxe2x88x922, using a patterned photoresist layer as mask, to produce source/drain regions 207, 207xe2x80x2 showing an impurity concentration level of about 1019 cmxe2x88x923 (FIG. 23B).
Then, a PSG layer 208 is formed on the entire surface of the substrate 201 as interlayer film. The PSG 208 may be replaced by NSG (Non-doped Silicate Glass)/BPSG (Boro-Phospho-Silicate Glass) or TEOS (Tetraethoxy-Silane). Thereafter, a contact hole is formed by patterning in the PSG 208 at a position right above the source/drain regions 207, 207xe2x80x2 and then an Al layer is deposited by evaporation, using a sputtering technique, and then patterned to produce an Al electrode 209 (FIG. 23C). Desirably, a barrier metal layer such as a Ti/TiN layer is formed between the Al electrode 209 and the source/drain regions 207, 207xe2x80x2.
Thereafter, a plasma SiN layer 210 and then a PSG layer 211 are formed on the entire surface of the substrate 201 to respective thicknesses of about 3,000 angstroms and 10,000 angstroms (FIG. 23D).
Then, The PSG layer 211 is patterned, using the plasma SiN layer 210 as dry etching stopper layer, until it is left only on the pixel separating regions and subsequently a through hole 212 is formed by patterning, using a dry etching technique, right above the Al electrode 209 that is held in contact with the drain region 207xe2x80x2 (FIG. 23E).
Thereafter, a pixel electrode 213 is formed to a film thickness of more than 10,000 angstroms by sputtering or EB (electron beam) evaporation (FIG. 24F). The pixel electrode 213 is typically made of film of a metal selected from Al, Ti, Ta and W or a compound of any of them.
Then, the surface of the pixel electrode 213 is polished by CMP (FIG. 24G).
An oriented film 215 is formed on the active matrix substrate prepared by the above described process and the surface of the oriented film is subjected to an orientation process which is typically a rubbing process before the substrate is bonded to an opposite substrate with spacers (not shown) interposed therebetween and liquid crystal 214 is filled into the gap to produce liquid crystal devices (FIG. 24H). Note that the opposite substrate comprises color filters, a black matrix, a common electrode 223 typically made of ITO and an oriented film 215xe2x80x2 arranged on a transparent substrate 220.
The reflection type liquid crystal device is typically driven in a manner as described below. A signal potential is applied to the source region 207 from a peripheral circuit such as shift register formed on the substrate 201 on an on-chip basis and, simultaneously, a gate potential is applied to the gate electrode 205 to turn on the switching transistor of the pixel and feed the drain region 207xe2x80x2 with a signal charge. The signal charge is stored in the depletion layer capacitance of the pn-junction formed between the drain region 207xe2x80x2 and the PWL 203 and provides the pixel electrode 213 with a potential by way of the Al electrode 209. The potential application to the gate electrode 205 is stopped to turn on the pixel switching transistor when the potential of the pixel electrode 213 gets to a desired level. Since the signal charge is stored in the capacitance of the pn-junction as described above, the potential of the pixel electrode 213 is stabilized until the pixel switching transistor is driven next time. Then, the liquid crystal sealed in the gap between the substrate 201 and the opposite substrate 220 as shown in FIG. 24H is driven by this stabilized potential of the pixel electrode 213.
As for the above described active matrix substrate, since the surface of the pixel electrode 213 is plane and smooth and an insulation layer is buried between any two adjacently located pixel electrodes as shown in FIG. 24H, the surface of the oriented film 215 formed thereon is also smooth and free from undulations. Therefore, the problems of known devices attributable to such surface undulations including a reduced light utilization efficiency due to scattered incident light, a poor contrast due to defective rubbing and the generation of bright lines due to a transversal electric field generated by the steps among the pixel electrodes are eliminated to improve the quality of the displayed image.
However, it has become apparent that a reflection type liquid crystal display apparatus of the above described patent document lacks deliberate considerations on the drive circuit of the active matrix substrate and, according to a study of the inventor of the present invention, has much room for improvement. More specifically, regardless if horizontally or vertically, the timing of operation of the CMOS transfer gate switch that is turned on and off by the output of the shift register is not considered for the display apparatus of the above patent document. If the timing of turning off the pMOS transistor and that of turning off the nMOS transistor in a CMOS transfer gate switch operating as sampling switch, the feedthrough of the one that is turned off late can remain and interfere with the operation of accurately transferring the voltage.
Additionally, for producing a plurality of chips from silicon wafers simultaneously for a liquid crystal apparatus comprising a plurality of liquid crystal devices and peripheral circuits, the space occupied by the peripheral circuits should be minimized.
However, it has become clear with known manufacturing techniques that, in order to make the timing of turning off the pMOS and that of turning off the nMOS transistor agree with each other, measures have to be taken including the provision of a feedback circuit for equalizing the time required for the shift to the H level side to the time required for the shift to the L level side and/or that of a anti-feedthrough circuit. Such measures can significantly raise the circuit dimensions and, additionally, the channel width (W) of the pMOS transistor will have to be designed to be much greater than that of the nMOS transistor according to the moving rate of the transistors to consequently raise the area occupied by the peripheral circuits. As a result of intensive research efforts of the present invention, it has been found that the above identified problems can be dissolved without using a large area for the peripheral circuits by making both the pMOS transistor and the nMOS transistor show an identical turning-off performance for the CMOS transfer gate switch.
Therefore, it is an object of the present invention to provide a liquid crystal display apparatus that can display high quality images without using a large area for the peripheral circuits by making both the pMOS transistor and the nMOS transistor show an identical turning-off performance for the CMOS transfer gate switch.
Another object of the invention is to provide a liquid crystal display apparatus comprising one or more than one shift registers, characterized in that the output of at least one of the shift registers is connected to the gate of the CMOS transistor by way of an inverter and the timing of operation of the input gate of the inverter connected to the gate of the pMOS transistor of said CMOS transistor is earlier than that of the input gate of the inverter connected to the gate of the nMOS transistor of said CMOS transistor.
According to the invention, the above object is achieved by providing a liquid crystal display apparatus of the type under consideration, wherein the timing of turning off the input gate of the inverter connected to the gate of the pMOS transistor of the CMOS transistor is earlier than that of the input gate of the inverter connected to the gate of the nMOS transistor of the CMOS transistor by the difference between the two MOS transistors in the time required for getting to a threshold level after turning off the input gate.
According to the invention, there is also provided a liquid crystal display apparatus comprising one or more than one shift registers, wherein said shift registers are horizontal shift registers and the outputs are connected to the CMOS transistor for sampling video signals by way of respective inverters.